Fluorescein injection following acute coronary occlusion caused the ischemic area of myocardium to appear dark against brightly fluorescent surrounding tissue. The border between normal and ischemic tissue was always clearly defined. Coronary occlusion always caused an increase in NADH fluorescence in the ischemic areas. Simultaneous changes in 529 nM fluorescence in these areas suggested an increase in the blood volume of acutely "ischemic" myocardium. These techniques may have a broad applicability in human coronary vascular surgery, e.g., to evaluate the effectiveness of coronary bypass grafts.